1. Field of the Invention
The present invention relates to an information processing apparatus, an information processing method, and a storage medium.
2. Description of the Related Art
Examinations and therapy based on capturing a moving image using radiation (e.g., X-rays) are actively performed in the medical field, and recently, X-ray imaging apparatuses that use a flat panel detector have come to be used frequently. Due to arranging photodiodes in an array, these flat panel detectors realize the flattening of an X-ray detection surface and resolve the problem of electro-optical warping that occurred with X-ray imaging apparatuses that use conventional image intensifiers. However, since flat panel detectors read, through long signal lines, signals photoelectrically converted by photodiodes, noise readily occurs in images due to the influence of external and internal factors.
Additionally, when capturing an image using radiation, low-dose capturing is required in order to reduce bodily exposure. Because of this, the signal to be read has an extremely small value, and even small fluctuations that occur in the image are visible. In particular, the human eye can acutely detect streaky unevenness (hereafter referred to as linear noise) that runs vertically and horizontally, and thus, the noise has a large effect on image quality during examination and therapy.
Conventionally, a technique using a spatial filter is known as a technique of reducing linear noise, as indicated in Japanese Patent Laid-Open No. 2011-028588. Japanese Patent Laid-Open No. 2011-028588 describes a technique in which the weighted average of an original image that includes linear noise is calculated to obtain an image having reduced random noise, and linear noise is extracted from the obtained image using a non-linear high pass filter. Then, a method is described in which the linear noise is reduced by subtracting the linear noise from the original image.
Additionally, Japanese Patent Laid-Open No. 2011-134118 discloses a method in which the weighted average of an original image that includes linear noise is calculated so as to obtain an image having reduced random noise, and then linear noise is reduced by linearly combining the obtained image with the original image and performing recursive processing in the time direction.
Additionally, Japanese Patent No. 4072491 discloses a method in which noise is reduced after decomposing an image into multiple frequency bands.
Japanese Patent Laid-Open No. 2011-028588 discloses a technique in which spatial filter processing is implemented on linear noise, which causes the linear noise to be reduced. In this case, the spatial filter can hold only a limited number of taps. Accordingly, it is difficult to handle low-frequency linear noise in spatial frequency bands at the same level as the object. As a result of this, low-frequency undulations remain in an image, which appear as flickering in a moving image.
Japanese Patent Laid-Open No. 2011-134118 discloses a technique in which recursive processing is performed in the direction of time, and therefore, it is possible to reduce low-frequency linear noise. However, since it takes time for a recursive filter to stabilize, it is difficult to reduce linear noise in the first 10 or so captured frames. Additionally, the technique disclosed in Japanese Patent Laid-Open No. 2011-134118 cannot be applied to still image capturing.
Japanese Patent No. 4072491 describes a technique in which reduction of random noise is performed by decomposing an image into multiple frequency bands. This technique enables the reduction of random noise in all frequency bands due to performing noise reduction processing on each frequency band. However, since it is not a technique that targets linear noise, it has little effect on reducing linear noise.
Additionally, as a person skilled in the art can easily envision, it is possible to reduce linear noise by performing filter processing after converting an image into frequency domains using high-speed Fourier transformation or the like. With this method, it is possible to perform processing of all frequency bands, but positional information of the image is lost when the image is converted into frequency domains. Since a non-linear filter such as an ε filter cannot be used as a result of this, it is difficult to separate the object and the noise.
The present invention provides a technique that can reduce linear noise in all spatial frequency bands in one image that is not readily influenced by random noise, and that does not readily influence the object.